Here’s our story, so far:
I speak English and decent “street” German. Mom speaks English and Thai, both pretty much at a “native” level. Mom started out trying to speak Thai at home with our daughter, but is not a particularly patient person and would switch to English sometimes and when our daughter began to speak her first words were English (and “Dada”, BTW) so she gave up fairly early and spoke mostly English to her. My son was born when my daughter was 27 months old. AFAIK my wife did not attempt to speak Thai to him, at least not often.
Then, when my daughter was 30 months and my son 3 months, we moved to Geneva. We lived in Geneva for about 5-6 months. At the end of this time my daughter could speak some French, and her pronounciation was far better than mine. Neither of us parents picked up French to the point of even being conversational.
From Geneva, we moved to Germany, where we remained for about 2 years. At this point my daughter began to attend school, but to my utter disappointment they did not teach German to the children in the international school until they get to around 6th grade. Stupid, IMO. I found some children’s books in German and they would see various TV programs in German. They only had a little bit of interaction with German children, as German people do not interact with strangers very much and the children pretty much follow suit. Both my son and daughter learned some German, as I found when I was watching the Simpsons one day and my daughter started asking questions about the storyline. (This caused me to stop watching the Simpsons in front of her) They were particularly fond of words that begin with the “sch” sound, like Schnecken and Schmetterling (snail and butterfly).
Then we moved to Thailand. We got a nanny from Isaan, who spoke Thai and Isaan (a dialectic mix of Thai and Laos) with the kids. They started to get more Thai at home. At my insistence we hired a tutor to come by and teach the kids “play Thai”, eg, conversation which a child would use. Again Thai was not taught to young children at the international school. In fact, Thai was banned since most of the children at this particular school were from wealthy Thai families, so they got Thai at home and the parents were paying a premium to get their children fluent in English. Still they picked up Thai much more quickly than French or German, mainly because of the home environment.
Epilogue: we have moved back to the USA since three months ago. Mom still reads some bedtime stories to the kids in Thai but is again wandering away from using it at home. French is a distant or forgotten memory for them. They recall some German, and do know if someone is speaking German, but I imagine it will fade quickly as well. The other day my daughter, who is now almost 6 years old, told me that she doesn’t want to “be Thai”. Note that we live in Cincinnati that has more than it’s fair share of blonde haired, blue eyed little children of Germanic descent. The girl has been pretty big into fitting since seemingly forever. She told her mother the other day that she is half German since her father is German (I am not). This is her way of confusing the language with the national identity.
Part of the bitter irony of all this is that she likes German people better because Thai people “laughed at her”. In reality most Germans reacted stoically or with a slight smile when they met her. Thai people on the other hand found her extremely beautiful and engaging and would utter a polite little laugh of pleasure when meeting her. Since she didn’t understand, at least initially, that they were saying “how beautiful, how lovely” when they met her she assumed they were laughing at her, since the Germans never laughed. My son, partially by virtue of being younger, doesn’t particularly have this aversion.